Asynchronous systems have been widely used in recent years for many purposes, including communication between a computer and peripheral devices. One prevalent use has been in communication with a computer of relatively low processing capability, known as a personal computer. Such a personal computer typically includes hardware such as a microprocessor, and its own software. It is desirable in certain applications to enable this computer to selectively communicate with one of a plurality of devices. A device which does this is known as a multiplexer.
However, the only stand-alone general purpose multiplexers which have been previously used, have been systems in which a communication with an external device could only occur based on a manual command. Therefore, to use such a multiplexer, the user of the computer would have to manually switch a switch command device to connect the computer to the desired peripheral device. Although there have been some multiplexers in the prior art which performed automatic switching functions, these multiplexers have typically been incorporated in a computer or in other such devices, and have not been independent general purpose units.
Moreover, a desirable conventional system includes many peripheral devices connected to a computer. Printers and CRT terminals are typical examples of such devices. Other such devices include modems, testers for performing Go/No Go tests using a microprocessor carrier board, an adapter for allowing the personal computer to be used as a logic analyzer, and the like. Therefore, personal computers have been increasingly interfaced with such peripheral devices.
In addition, there has been a demand in recent years for a polling-type connection to such devices. Such a polling connection would enable a peripheral device, such as a printer, to operate in a polling mode. In this mode of operation, the computer could send enough information to the print buffer of the printer to enable the printer to print. Another transmission to the print buffer could be requested by the printer after it has printed the contents. The general purpose type multiplexers of the prior art cannot perform such a function.
To meet the above objects and to overcome all the problems which have existed in the prior art, the present invention describes a general purpose device with a highly configurable selector switch box. This box is usable for connection between devices of a system such as described above. The device is adapted to be connected between a first terminal that comprises an RS232C interface. A plurality N of second terminals are also provided, each of which also includes an RS232C interface. Each of the second terminals, as well as the first terminal, is connected to a switching circuit. This switching circuit performs the function of selectively connecting at least one of the second terminals to the first terminal.
A command analyzing circuit is also connected to the first terminal. This command analyzing circuit receives one of the RS232 signal lines from the command analyzing circuit. Typically, the signal which would be received is the data terminal ready (DTR) signal or the request to send (RTS) signal. After receiving DTR or RTS, the command analyzer receives a channel designation number and/or a polling mode number contained within the data on the transmit data (TXD) line.
The output of the command analyzing circuit is therefore a specific channel designation including either a mode number or a polling mode number. The output of this signal is coupled to an auto/manual circuit. This auto/manual circuit is a switch which enables one of two input signals to be switched t its output. One of the input two signals is the channel designation number from the command analyzer. At the other input, the auto/manual switch receives an output from a manual channel select switch. This manual channel select switch enables a manual control of such channel selecting.
The output of the auto/manual switch is connected to a decoder which translates the parallel format command signal to a plurality of discrete output signals. These discrete output signals are connected either to the connecting circuit (via an OR gate discussed later) or to a scanning circuit.
The scanning circuit is also connected to the RS232 signals from each of the second terminals. The scanning circuit is responsive to an output from the decoding circuit, and polls signals from the second terminals in response to an appropriate output from the decoding circuit. The typical signals which would be polled are the data set ready (DSR) signal and the carrier detect (CD) signal. The output of the polling circuit is another channel designation number, representing a number of the channel found active during polling. The channel designation number from the decoder and from the polling circuit are both connected to OR gate 36. When either one of these signals is present, the connecting circuit is commanded to make an appropriate connection.
A baud rate generator 32 is also provided for clocking and synchronizing the entire system.
The invention is advantageous for the following reasons. Since the device is compact and integrally constructed, and uses general purpose type interfaces, it can be applied to a large number of devices. Due to its ability to be structured in a single box and switchable based on a command from an external terminal, less user intervention is required. Switching control can be conducted either manually or automatically. The circuit construction is relatively simple. Furthermore, the device can be easily made using standard available TTL type logic or the like, and can be used for a number of purposes.